Dienophile

A dienophile can only properly be explained by being familar with a Diels-Alder reaction. In a classic diels-alder reaction one molecule has two conjugated double bonds example 1,3-butadiene, and this reacts with molecule with one double bond example 1-propene. The first component is called the diene and the second is called the dienophile. reactionsceme THIS DOES NOT MEAN THAT ALL MOLECULES WITH ONE DOUBLE BOND ARE DIENOPHILES. Only when a molecule readily reacts with a molecule that has two conjugated double bonds in a diels-alder manner, is it considered a dienophile. It is worth noting that the amount of cyclic rings present in the product is always one more than was present totally in the reactants.

Diels-Alder

The Diels-Alder reaction is an organic chemical reaction (specifically, a cycloaddition) between a conjugated diene and a substituted alkene, commonly termed the dienophile to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly-formed ring are not carbon. Some of the Diels-Alder reactions are reversible; the decomposition reaction of the cyclic system is then called the Retro-Diels-Alder. carbon This reaction belongs to a larger class known as [2 + 4]cycloadditions, which are characterized by the formation of a ring by a process involving two pi electrons of one reactant and four pi electrons of the other. Such reactions tend to be especially rapid because the transition state (see activation energy) involves six pi electrons delocalized around a ring, much like benzene. It therefore has some aromatic character and is particularly stable. By comparison, [2 + 2] and [4 + 4] cycloadditions tend to be much slower, as the transition state is anti-aromatic. These reactions can be performed photochemically.

The mechanism

The reaction occurs via a single transition state which has a smaller volume than either the starting materials or the product. It is an associative type of reaction, and it is speeded up by very high pressures. Diels-Alder is an example of a pericyclic reaction. Some free radical versions of this reaction have been observed, these are not Diels-Alder reactions as the stereochemistry at the carbons is scrambled. These are step wise reactions of the free radicals which form the new bonds in at least two steps. An example of this type of reaction is the reaction of selenobenzopheone with a 1,3-diene. Please see the entry on thioketones for more details.

The diene

The diene component in the Diels-Alder reaction can be open-chain or cyclic and it can have many different kinds of substituents. There is only one limitation: it must be able to take up the cis-conformation. Butadiene normally prefers the s-trans conformation with the two double bonds as far away from each other as possible. The reason for this is that trans conformation allows the electrons from each double bond to very slightly overlap and lower the overall molecular orbital of butadiene. If there are substituents larger than hydrogen then a butadiene derivative may conform to a trans conformation for steric reasons. The barrier to rotation about the central bond is small and rotation to the less favourable but reactive s-cis conformation is rapid. Cyclic dienes that are permanently in the s-cis conformation are exceptionally good at Diels-Alder reactions (cyclopentadiene is a classic example), but cyclic dienes that are permanently in the s-trans conformation and cannot adopt the s-cis conformation will not do the reaction at all. Dendralenes are a new class of experimental DA dienes. Unstable dienes, such as quinodimethane, can be generated in situ. Aromatic stabilization in the product of a DA reaction using such a diene is, in some cases, the reason behind the very high reactivity and lack of stability of such diene. The use of such unsable diene is advantageous, despite the trouble, in that the products will contain newly formed aromatic six-membered rings. Benzenoid compounds rarely undergo DA reactions and often require very reactive dienophiles.

The dienophile

In a typical Diels-Alder reaction, the dienophile has an electron-withdrawing group conjugated to the alkene. Though common, this feature is not exclusive of Diels-Alder dienophiles. There must be some extra conjugation, at least a phenyl group or chlorine atom. The dienophile can be activated by a Lewis acid such as niobium pentachloride . niobium pentachloride Cyclopentadiene does not react with cyclohexenone in ethyl acetate unless the Lewis acid is present. The yield improves when reaction temperature is lowered to -78°C because polymerization side reactions are prevented. Niobium pentachloride catalysis gives only the endo conformer. The same reaction with aluminium chloride results in a endo and exo mixture. Many of these lewis acids are not good catylists for the reaction of alpha,beta-unstaurated carbonyls, this is because the carbonyl oxygen binds too tightly to the metal centre. A far better catylist for such a system is a combination of silver perchlorate and lawesson's reagent in cold dichloromethane Otto Paul Hermann Diels and Kurt Alder were awarded the Nobel Prize in Chemistry in 1950 for their work on this reaction. The Diels-Alder reaction is generally considered the "Mona Lisa" of reactions in organic chemistry since it requires very little energy to create the very useful six-membered ring. It is well known that it is possible to use heteroatom containing dienophiles for Diels-Alder reactions, for instance Lawesson's reagent ( and diferrocenyl dithiadiphosphetane disulfide) can react with 1,3-dienes to form six membered ring adducts. Also selenoketones and thioketones are able to react in the same way with 1,3-dienes. Dienophiles can be chosen to contain a "masked functionality". The dienophile undergoes Diels-Alder reaction with a diene introducing such a functionality onto the product molecule. A series of reactions then follow to transform the functionality into a desirable group. The end product cannot not be made in a single DA step because equivalent dienophile is either unreactive or inaccessible. An example of such approach is the use of alpha-chloroacrylonitrile (CH2=CClCN). When reacted with a diene, this dienophile will introduce alpha-chloronitrile functionality onto the product molecule. This is a "masked functionality" which can be then hydrolyzed to form a ketone. Alpha-chloroacrylonitirle dienophile is an equivalent of ketene dienophile (CH2=C=O), which would produce same product in one DA step. The problem is that ketene itself cannot be used in Diels-Alder reactions because it reacts with dienes in unwanted manner (by [2+2] cycloaddition), and therefore "masked functionality" approach has to be used. (Ranganathan, Synthesis, v1977, p289) Other such functionalities are phosphonium substituents (yielding exocyclic double bonds after Wittig reaction), various sulfoxide and sulfonyl functionalities (both are acetylene equivalents), and nitro groups (ketene equivalents).

Heterodienophiles

No major loss in reactivity of dienophile is seen when one, or both, of the carbons are substituted for another variety of atom. Carbonyl groups, for example, can successfully react with dienes to yield pyranoid rings. Generally, the endo transition state is favored in this case. Nitroso compounds (N=O) react to form oxazine-like compounds (cyclic molecules with nitrogen and oxygen present in the six-membered ring). Another group of dienophiles successfully used for DA reactions is imines. Such reactions are useful for preparation of alkaloid and other polycyclic compounds.

Stereoselectivity in DA Reactions

Diels-Alder reactions can lead to formation of a variety of structural isomers and stereoisomers (enantiomers and diastereomers). Identity of major products can usually be predicted, however. In unsymmetrically substituted diene and dienophile, ortho and para orientations in products are usually favored over meta orientation. A particular preference in location of substitutents in the product can, in some cases, be explained in terms of frontier orbital theory. Most commonly, diene bears an electron-releasing group (ERG) and dienophile bears an electron-withdrawing group (EWG). The strongest interaction takes place between HOMO of diene and LUMO of dienophile. Carbons that have the highest coefficients in two frontier orbitals will begin to bond; therefore these carbons will direct the orientation of substituens and thus identity of major product of a DA reaction. Dealing with the actual frontier orbital coefficients can be avoided since the preferred orientation in product can be described in terms of partial positive and negative charges that exist in diene and dienophile. Carbon with a partial negative charge will interact most readily with carbon bearing a partial positive charge. Therefore those two carbons will start coming together, thus dictating the relative orientation of substituents. The existence of partial positive/negative charge can always be determined by drawing resonance contributors for diene and dienophile, taking their ERG and EWG into consideration. LUMO According to the 'cis principle' formulated by Alder and Stein in 1937, the stereochemistry of substituents in the starting material is retained in the product. This means that if a cis-dienophile is reacted, both of the cis-substituents will end up on same side (face) of the product ring. Trans-dienophile will yield a product where both of trans-substituents (that came originally from the dienophile) will be on different sides of the product ring. The same principle applies to dienes. Trans,trans 1,4-substituents will end up on same side of the ring, whereas trans,cis 1,4-substituents will be oriented towards different faces of the ring. Besides the ortho/meta/para-forming orientations, the diene and dienophile may arrange themselves in different ways to yield exo and endo transition states which result in different products. To determine which is the endo and which is the exo transition state, the two molecules are oriented parallel to each other, such that diene's single bond (one which connects two double bonds) is parallel to dienophile's double or triple bond. It makes no difference whether the dienophile is positioned above or below the diene. The single substituent (or cis-substituents on the dienophile) is oriented to point in the directon of diene's pi-system. This is the endo transition state (pictured below). If these substituents are pointed away from the diene, this would be the exo transition state. Using the 'cis principle' it is understood that cis-substituents on dienophile, for example, will end up on same side of the molecule. It is not obvious where the substituents on both diene and dienophile will end up relative to each other. Will they end up cis ot trans to one another in the product molecule? To predict the orientation of these substituents that are coming from different molecules the different transition states have to be examined. The most stable transition state will lead to the major product. Transition state will also dictate the relative orientation of diene's and dienophile's substituents on the product ring. In some cases another rule can be applied: the 'endo addition rule'. According to this rule, the most stable transition state results when there is a 'maximum accumulation of double bonds'. This rule is not always followed. It has most application when dealing with cyclic dienes and dienophiles. For example, DA reaction of cyclopentadiene and maleic anhydride yields over 95% of the endo product. It is important to note that labels "exo" and "endo" relate to the orientation of substituents in the transition state and not to a specific orientation of substituents in the product molecule. In each individual case, the transition state has to be examined to see the most favored relative orientation of substituents. It is not the case that for endo transition state the substituents on dienophile and 1,4-substituents on diene always point towards the same side of the newly formed ring. "Endo" and "exo" define specific transition states, not orientation of substituents. In the picture below, it just happens that the endo transition state will yield substituents on same side of the ring. This is not always so. In case of maleic anhydride and cyclopentadiene the endo product will have the R groups of diene and dienophile oriented toward the opposite sides of the newly formed ring. diene diene Exo product can predominate, however, for some reactions. This can happen when the resulting endo product can easily dissociate back into the starting material. In such reactions, exo product predominates over extended reaction times because exo product is thermodynamically favored. In other cases, endo product can convert to what would be the exo product of the reaction. In the example below, endo product B was the only one isolated after Diels-Alder reaction. However, letting the reaction go for prolonged periods of time also yielded substantial amounts of exo product A. The authors speculated that endo product B can epimerize to exo product A in the following way: epimer In summary, diastereoselectivity is based on the postulation of transition state. For any given DA reaction, one can imagine one possible transition state being favored over the other due to steric, stereoelectronic, and complexing factors. Thus, predictions can be made on the identity of major product of a particular DA reaction by looking a the starting material available. Diels-Alder reactions lend themselves to chiral synthesis with chiral auxiliaries or chiral ligands. In one research effort the auxiliary is derived from L-asparagine. The telescopic synthesis with cyclopentadiene and acrylic acid yields the DA adduct with three stereocenters as predominantly the endo conformer and with 54% ee. ee Lewis acids (AlCl3, ZnCl2, and others) act as catalysts by coordinating to the dienophile. The complexed dienophile becomes more electrophilic and more reactive toward the diene. This increases the rate and often the stereoselectivity of a DA reaction.
External links

- [http://photoz.atspace.com/reaction.htm Diels-Alder Reactions @ photoz.atspace.com]
References
# Niobium Pentachloride Activation of Enone Derivatives: Diels-Alder and Conjugate Addition Products Mauricio Gomes Constantino, Valdemar Lacerda Júnior and Gil Valdo José da Silva Molecules 2002, 7, 456–465 [http://artemis.ffclrp.usp.br/PublicaPDF/Valdemar_Mol02.pdf Online article] open access publication # A fully-telescoped, aqueous, auxiliary-mediated asymmetric transformation Mathew P. D. Mahindaratne, Brian A. Quiñones, Antonio Recio III, Eric A. Rodriguez, Frederick J. Lakner,1 and George R. Negrete Arkivoc(EJ-1566C) pp 321-328 2005 [http://www.arkat-usa.org/ark/journal/2005/I06_Juaristi/1566/EJ-1566C.asp Online Article] Category:Organic reactions

1,3-Butadiene
1,3-butadiene is a simple conjugated diene having the chemical structure shown at right. It is an important industrial chemical used as a monomer in the production of synthetic rubber. When the word butadiene is used, most of the time it refers to 1,3-butadiene. The name butadiene can also refer to the isomer, 1,2-butadiene, which is a cumulated diene. However, this allene is difficult to prepare and has no industrial significance.
History
In 1863, a French chemist isolated a previously unknown hydrocarbon from the pyrolysis of amyl alcohol. This hydrocarbon was identified as butadiene in 1886, after Henry Edward Armstrong isolated it from among the pyrolysis products of petroleum. In 1910, the Russian chemist Sergei Lebedev polymerized butadiene, and obtained a material with rubber-like properties. This polymer was, however, too soft to replace natural rubber in many roles, especially automobile tires. The butadiene industry originated in the years leading up to World War II. Many of the belligerent nations realized that in the event of war, they could be cut off from rubber plantations controlled by the British Empire, and sought to remove their dependence on natural rubber. In 1929, Eduard Tschunker and Walter Bock, working for IG Farben in Germany, made a copolymer of styrene and butadiene that could be used in automobile tires. Worldwide production quickly ensued, with butadiene being produced from grain alcohol in the Soviet Union and the United States and from coal-derived acetylene in Germany.
Production
In the United States, western Europe, and Japan, butadiene is produced as a byproduct of the steam cracking process used to produce ethylene and other olefins. When mixed with steam and briefly heated to very high temperatures (often over 900 °C), aliphatic hydrocarbons give up hydrogren to produce a complex mixture of unsaturated hydrocarbons, including butadiene. The quantity of butadiene produced depends on the hydrocarbons used as feed. Light feeds, such as ethane, give primarly ethylene when cracked, but heavier favor the formation of heavier olefins, butadiene, and aromatic hydrocarbons. Butadiene is typically isolated from the other four-carbon hydrocarbons produced in steam cracking by extraction into a polar aprotic solvent such as acetonitrile or dimethylformamide, from which it is then stripped by distillation.
From ethanol
In other parts of the world, including eastern Europe, China, and India, butadiene is also produced from ethanol. While not competitive with steam cracking for producing large volumes of butadiene, lower capital costs make production from ethanol a viable option for smaller-capacity plants. Two processes are in use. In the single-step process developed by Sergei Lebedev, ethanol is converted to butadiene, hydrogen, and water at 400–450 °C over any of a variety of metal oxide catalysts: 2 CH₃CH₂OH → CH₂=CH-CH=CH₂ + 2 H₂O + H₂ This process was the basis for the Soviet Union's synthetic rubber industry during and after World War II, and it remains in limited use in Russia and other parts of eastern Europe. In the other, two-step process, developed by the Russian chemist Ivan Ostromislensky, ethanol is oxidized to acetaldehyde, which reacts with additional ethanol over a tantalum-promoted porous silica catalyst at 325–350 °C to yield butadiene: CH₃CH₂OH + CH₃CHO → CH₂=CH-CH=CH₂ + 2 H₂O This process was used in the United States to produce Government Rubber during World War II, and remains in use today in China and India.
Uses
Most butadiene is polymerized to produce synthetic rubber. While polybutadiene itself is a very soft, almost liquid material, polymers prepared from mixtures of butadiene with styrene or acrylonitrile, such as ABS, are both tough and elastic. Styrene-butadiene rubber is the material most commonly used for the production of automobile tires. Smaller amounts of butadiene are used to make nylon via the intermediate adiponitrile, other synthetic rubber materials such as chloroprene, and the solvent sulfolane.
Safety
Contact with liquid butadiene can result in irritation of the skin, eyes, and mucous membranes. Since it often stored as a refrigerated liquid, frostbite is another possible consequence of exposure. When inhaled, butadiene is a mild depressant and can result in drowsiness, although very high concentrations are necessary to produce unconsciousness or death. In some animals, long-term exposure to butadiene can result in cancer of the liver or kidneys. Butadiene is a potent carcinogen in mice, but only a weak carcinogen in rats. Studies of workers in chemical plants using butadiene have shown no conclusive increase in cancer risk for whatever amount of butadiene these workers may have been exposed to, so butadiene remains classified as only a potential human carcinogen.
References
# Armstrong, H.E. Miller, A.K. (1886). "The decomposition and genesis of hydrocarbons at high temperatures. I. The products of the manufacture of gas from petroleum." Journal of the Chemical Society 49, 80. # Caventou, E. (1863). Annalen der Chemie und Pharmacie 127, 93. # Kirshenbaum, I. (1978). Butadiene. In M. Grayson (Ed.), Encyclopedia of Chemical Technology, 3rd ed., vol. 4, pp. 313–337. New York: John Wily & Sons. # Sun, H.P. Wristers, J.P. (1992). Butadiene. In J.I. Kroschwitz (Ed.), Encyclopedia of Chemical Technology, 4th ed., vol. 4, pp. 663–690. New York: John Wiley & Sons. Category:Dienes Category:Monomers ja:ブタジエン

Diene
Dienes are hydrocarbons which contain two double bonds. Dienes are intermediate between alkenes and polyenes.
Classes
Dienes can divided into three classes: #Unconjugated dienes have the double bonds separated by two or more single bonds. #Conjugated dienes have conjugated double bonds separated by one single bond #Cumulated dienes have the double bonds sharing a common atom as in a group of compounds called allenes. In organic chemistry a conjugated diene is also a functional group.
Common dienes
The simplest conjugated diene is 1,3-butadiene. Cyclopentadiene is another example of a diene. Cyclopentadiene
Reactions with dienes
The 1,3 configuration of double bonds found in 1,3-butadiene (conjugated double bonds) make these types of dienes capable of participating in more reaction types than is the case for molecules with either just a single alkene functional group or with multiple, but non-alternating, alkene groups. One possible reaction for such dienes is the Diels-Alder reaction. Category:functional groups Category:Hydrocarbons
Polonaise (Tanz)
Die Polonaise (von franz. danse polonaise = polnischer Tanz, ital. als Polacca und poln. als Polonez bezeichnet), laut Duden auch Polonäse, ist ein feierlich geschrittener Tanz im 3/4-Takt, der sich in dieser Form nach 1700 durchsetzte. Im 16. Jahrhundert noch im 4/4-Takt von den polnischen Würdenträgern als eine Huldigung an Polen getanzt, verbreitete er sich durch Heinrich III. zunächst in Frankreich und danach in ganz Europa. Johann Sebastian Bach schrieb stark stilisierte Polonaisen, während jene von Ludwig van Beethoven, Carl Maria von Weber, Franz Liszt und Frédéric Chopins die ursprüngliche Beschwingtheit und Würde wieder zur Geltung bringen. Heute wird die Polonaise oft als Einleitung von Bällen getanzt. Das hierbei am häufigsten Verwendung findende Musikstück ist die "Fächerpolonaise" von Carl Michael Ziehrer. Die Polonaise, der auch ein Stimmungslied gewidmet wurde ("Polonäse Blankenese" von Werner Böhm alias Gottlieb Wendehals), ist auch fester Bestandteil von Faschingsveranstaltungen. Siehe auch: Historischer Tanz Kategorie:Historischer Tanz

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